cu advice gladly needed ......

[ez2own]

Right gentlemen need to ask your advice , thanks in advance ..

Going to change cu to a 15 way with split rcd to bring up to 17th/part p , original board is 14 way with rcd protected on main switch only .
My only concern is small 4 way cu in garage ( if i rcd protect will possibly trip main board in house if anything went pear shaped , would i be best to leave small cu in garage with rcd protection , but place into house cu with rcd not connected to garage supply.
only supply garage has external is 2 lights on a motion sensor .or would no rcd in garage be a non comply ????

Thanks again ...

 

[GLENNSPARK]

But whereabouts does the fault occur tony?.......

 

[GLENNSPARK]

Seems you are the only one that knows, that your not relying on a ground to trip an RCD. having two separate ground points will make little to no difference at all ...lol!! But your house connection point on your drawing is in the wrong place. lol!!

Glennspark,

The ONLY way to ensure discrimination between similar rated RCDs, is by a time delay.
Iv'e known 30mA and 500mA having no discrimination between them...

Ok. and thank you eng....I will be sure to keep this in mind....

 

[Knobhead]

Hang on I’m shipping water here!

E54
As I read the OP the house would have a RCD feeding the main CU, off which a SWA would feed a second RCD protected CU. Sorry if I misread anything.

Glen
With the set up as in my drawing any fault in either the garage or house would trip the house first.

 

[SPARTYKUS]

This is in reply to Glen.

One thing you have to take in to account is there will be a slight leakage from the house circuits added to any leakage from the garage.

So say you have 10mA leakage in the house (PC’s, fridges Etc.) it’s effective range is now 20mA, where as the garage remains at 30mA.

The question remains the same, which trips first?

I'll put my neck on the block and say you can never truly say. Either or. Their individual characteristics are not guaranteed to remain steadfast as already mentioned many variables can affect, like ambient temperature (are they sited in different environments for example) , or as another possible when did they both last trip, I was taught that they do get a little sticky sometimes if not been tripped (either from a fault or by test button ) for a while and they pick up a little "lubrication" shall we call it when tripped.

Fix bayonet!

OP the other killer with the existing setup is when the garage is some distance from the house ... just done a similar thing at a property where the garage is 100M from the house. And yes, one RCD was covering the entire 2.5 acre site with 5 buildings dotted around ...... Arrrghhh

 

[Knobhead]

If anyone comes up with a cartoon of bailing a boat I’ll personally come and sort them out!

View attachment 8232

 

[Big n Daft]

I take it this is where individual RCBO's are most useful.................
and an isolator on the garage CU.........................just a thought................

 

[Big n Daft]

If anyone comes up with a cartoon of bailing a boat I’ll personally come and sort them out!

View attachment 8232

Wow.................. Now thats what I call a cattle prod

 

[Engineer54]

Hang on I’m shipping water here!

E54
As I read the OP the house would have a RCD feeding the main CU, off which a SWA would feed a second RCD protected CU. Sorry if I misread anything.

Glen
With the set up as in my drawing any fault in either the garage or house would trip the house first.

Just looks to me as the house connection, is shown halfway along the Sub-main to the garage....

I would say that there is more than an even chance that both RCDs will trip, both will in all probability see a tripping fault current. As i say there is only one sure way to ensure discrimination and that's via a time delay in the upstream device.

 

[topquark]

Just looks to me as the house connection, is shown halfway along the Sub-main to the garage....

I would say that there is more than an even chance that both RCDs will trip, both will in all probability see a tripping fault current. As i say there is only one sure way to ensure discrimination and that's via a time delay in the upstream device.

you are correct Eng, it should be taken below the diagram for the house rcd now above it, sorry tony, i looked but couldn't find a suitable cartoon

 

[DaveyD]

Tony? ......................

 

[Knobhead]

Can you fit a time delay RCD to protect the house just to give discrimination for the garage?

And the question is tongue in cheek!

 

[GLENNSPARK]

But my thinking was that if the garage feed was taken off an MCB and not an RCD then the garage board is given RCD protection for final circuits in the garage then surely this setup will not affect RCD protected circuits within the house....i didn`t mention inline RCDs....

 

[Knobhead]

View attachment 8236 Tony? ......................

Your at the top of the list


View attachment 8237

 

[topquark]

But my thinking was that if the garage feed was taken off an MCB and not an RCD then the garage board is given RCD protection for final circuits in the garage then surely this setup will not affect RCD protected circuits within the house....i didn`t mention inline RCDs....

that's what i'd do, as the house cu is being replaced anyway. Split board with rcbo/mcb on one neutral bar with the rcd covering the other, take the garage feed from an mcb. Alternatively as Nick suggested a high integrity board with 3 neutral bars.

 

[Knobhead]

If I changed this drawing, we could get some idea of where we’re coming from

View attachment 8238